There is a significant unmet need for effective therapies, particularly prophylaxis therapies, for migraine. Results from multiple studies indicate that ˜14 million migraineurs in the US could qualify and benefit from an effective and safe preventive therapy. Currently approved migraine prophylactic therapies are only partially effective and have considerable side-effect profiles which significantly limit the acceptability of these medications. Despite these limitations, 4.5 million individuals with frequent migraine headache in the US take prophylactic medication for their migraines.
Pituitary adenylate cyclase-activating polypeptides (PACAP) are 38-amino acid (PACAP38), or 27-amino acid (PACAP27) peptides that were first isolated from an ovine hypothalamic extract on the basis of their ability to stimulate cAMP formation in anterior pituitary cells (Miyata A, Arimura A, Dahl R R, et al. Isolation of a novel 38 residue-hypothalamic polypeptide which stimulates adenylate cyclase in pituitary cells. Biochem Biophys Res Commun. 1989; 164:567-574; Miyata A, Jiang L, Dahl R D, et al. Isolation of a neuropeptide corresponding to the N-terminal 27 residues of the pituitary adenylate cyclase activating polypeptide with 38 residues (PACAP38). Biochem Biophys Res Commun. 1990; 170:643-648). PACAP peptides belong to the vasoactive intestinal polypeptide VIP-secretin-hormone-releasing hormone (GHRH)-glucagon superfamily with the sequence of human PACAP27 shares 68% identity with vasoactive intestinal polypeptide (VIP) (Campbell R M and Scanes C G. Evolution of the growth hormone-releasing factor (GRF) family of peptides. Growth Regul. 1992; 2:175-191). The major form of PACAP peptide in the human body is PACAP38 and the pharmacology of PACAP38 and PACAP27 has not been shown to be different from each other. Unless indicated otherwise herein, PACAP or PACAP38 will be used to represent PACAP38, and PACAP27 will be used to specify PACAP27.
Three PACAP receptors have been reported: one that binds PACAP with high affinity and has a much lower affinity for VIP (PAC1 receptor, or simply “PAC1”), and two that recognize PACAP and VIP essentially equally well (VPAP1 and VPAC2 receptors, or simply “VPAP1” or “VPAC2”, respectively) (Vaudry D, Falluel-Morel A, Bourgault S, et al. Pituitary adenylate cyclase-activating polypeptide and its receptors: 20 years after the discovery. Pharmacol Rev. 2009; September 61(3):283-357).
As can be appreciated from the data in the table below, PACAP is capable of binding all three receptors with similar potency and is thus not particularly selective. VIP, on the other hand, binds with significantly higher affinity to VPAC1 and VPAC2, as compared with PAC1. In addition to endogenous agonists PACAP and VIP, maxadilan, a 65 amino acid peptide originally isolated from the sand-fly, is exquisitely selective for PAC1 compared with VPAC1 or VPAC2.